In the introduction to his autobiography, Mark Twain wrote that "The frankest and freest product of the human mind and heart is a love letter. The writer gets his limitless freedom of statement and expression from his sense that no stranger is going to see what he is writing." The writer is, of course, sorely deluded if he thinks any such thing. Literary history is crammed with the unguarded outpourings of lovers, spouses, lechers and romantics. And today, when one billion Valentine's Day cards will be sent worldwide, we might wonder where the tradition of the passionate avowal came from—and whether it's now dead and gone.